1. Field Of The Invention
The present invention pertains to a fracturing method for hydraulically fracturing an earth formation from a wellbore by overpressuring the wellbore with compressed gas so as to provide a high pressure flow condition during fracture initiation, and the invention particularly pertains to such methods which are suitable for use in deviated wells.
2. Description Of The Prior Art
In hydraulically fracturing earth formations to stimulate production of fluids therefrom, a long-standing problem has been the inability to sustain high pressure and flow of the fracturing fluid during fracture initiation. In deviated wells, in particular, inadequate pressure and flow conditions at the fracture initiation will produce multiple fractures and near wellbore kinks in the multiple fractures which will tend to restrict the flow of fluids to or from the wellbore once the fracture has been formed. U.S. Pat. No. 5,074,359 issued Dec. 24, 1991, to Joseph H. Schmidt and assigned to the assignee of the present invention discusses the problem of improper fracture formation from deviated wellbores, in particular. That patent is directed to a method for orienting the casing perforations to minimize improperly formed fractures at or near the perforations.
Conventional hydraulic fracturing is limited by the inability to provide the fracture fluid at sufficiently high initial flow rate and pressure to sustain the formation of a single slowly turning fracture. Since, in conventional fracturing techniques, the fracture fluid is supplied at the requisite pressure from surface disposed pumps, pressure losses through the pumping system and the wellbore conduits leading to the fracture zone often preclude adequate single fracture formation and extension during initial breakdown. The resultant pressure drop in the flow path in the improperly formed or inadequately extended fractures prevents or restricts the future ability to stimulate the formation with a hydraulic proppant frac.
One proposal which has been put forth to improve this situation is to overpressure the wellbore with compressed gas bearing on the fracturing fluid prior to formation of the perforations, so that immediately upon formation of the perforations, the expanding gas will force the fracturing fluid into the formation at or above formation breakdown pressure at a sustained flow rate for a sufficient time to provide a reduced number of fractures which will not pinch off or be kinked in the near wellbore region. Such techniques are suggested in U.S. Pat. No. 3,170,517 to Graham, U.S. Pat. No. 5,131,472 to Dees et al., and U.S. Pat. No. 5,271,465 to Schmidt et al.
Graham et al. and Schmidt et al. disclose the use of a wireline conveyed perforating gun. Dees et al. discloses alternatively the use of a tubing conveyed perforating gun disposed on the production tubing itself or the use of a wireline conveyed perforating gun.
Neither of these alternatives is desirable for use in deviated wells, especially where it is desired not to leave the perforating gun in the well after perforating. The wireline conveyed perforating gun cannot be run through deviated portions of a well which have such a significant horizontal component that gravity alone cannot pull the perforating gun down through the wellbore. Although a tubing conveyed perforating gun as suggested in Dees et al. could be run into a horizontal well, the perforating gun cannot be removed therefrom without pulling the entire production tubing string which is of course undesirable. Furthermore, if the perforating gun is of the type which is dropped upon firing, it could not be reliably dropped to the bottom of the borehole since it would likely remain in the deviated borehole portion.
As explained below, the present invention solves this problem with perforating methods which involve the use of a coiled tubing-string for running the perforating gun down through a production tubing string or through a casing. Unique techniques are provided for assuring the operability and integrity of the coiled tubing string in the unique environment in which it must operate, namely the extremely high pressure environment surrounding the coiled tubing prior to perforation.
Although the prior art does also include the use of coiled tubing conveyed tools in deviated boreholes such as shown for example in U.S. Pat. No. 3,401,749 to Daniel and U.S. Pat. No. 4,877,089 to Burns, the prior art has not previously shown or suggested methods for using such a system in the unique high pressure environment encountered in overpressured fracturing.